One of the goals of plant genetic engineering is to obtain plants having improved characteristics or traits. Many different types of characteristics or traits in plants are considered advantageous. Those of particular importance with regard to fruit bearing plants include control of fruit ripening, improvements in the nutritional characteristics of the edible portions thereof, resistance to plant diseases, resistance to insects, cold tolerance and enhanced stability or shelf-life of the ultimate consumer product obtained from the plant.
At least two key components are required to stably engineer a desired trait, or control of such a trait, into a plant. The first key component comprises identifying and isolating the gene(s) which either encode(s) or regulate(s) a particular trait. The second component comprises identifying and isolating the genetic element(s) essential for the actual expression and/or selective control of the newly isolated gene(s) so that the plant will manifest the desired trait and, ideally, manifest the trait in a controlled or controllable manner. This second component, which controls or regulates gene expression, typically comprises transcriptional control elements known as promoters. Although a generic class of promoters which drive the expression of heterologous genes in plants have been identified, a broad variety of promoters active in specific target tissues or cells of plants remain to be described. The identification of such target or tissue-specific promoters is critical to the introduction of the above-mentioned tissue-specific improvements in plants such as fruit bearing plants.
Several promoters useful in expressing heterologous genes in selected fruits have already been identified. For example, the E4 and E8 promoters (Deikman, et al.), the kiwifruit actinidin promoter (Lin, et al.) and promoter for polygalacturonase are known to be fruit specific. U.S. Pat. No. 4,943,674 (Houck et al., Jul. 24, 1990) discloses a 2All promoter as useful in expression of a heterologous gene in tomato fruit. These promoters, however, have been isolated from fruit tissue which comprises mature or maturing ovaries (hereinafter referred to as "traditional fruit"). As such, these traditional fruit promoters would be ineffective in controlling desired traits in such accessory fruit bearing plants as strawberry, apple, pear, quince and the like wherein the major portion of the edible fruit comprises receptacle tissue (see An Introduction to Plant Biology. 2nd Edition, Braungart & Arnett, eds., C.V. Mosby Co. 1965). Similarly, to date, genes thought to be active in fruit tissue have been isolated from traditional fruit tissue instead of receptacle containing tissue. Promoters involved in fruit expression have been identified in PCT application WO 97/27295.
There exists a need for receptacle tissue selective promoters which provide for increasing or decreasing expression during fruit development, maturatioin and ripening in the art. Access to such receptacle tissue selective promoters would enable the genetic engineering of fruit tissue from commercially important plants such as strawberry, apple, and pear. Two cDNAs have been previously identified as receptacle tissue selective (Reddy and Poovaiah, 1990, Plant Molecular Biology, 14: 127-136 and Wilkinson et al., 1995, Plant Molecular Biology 27:1097-1108). The promoters for these two cDNAs were cloned and sequenced. Expression of reporter genes in strawberry plants will be used as an assay of the tissue specificity of the isolated promoters. Of particular interest are promoters which provide for recepticle tissue selective expression of genes. Also of interest is the ability to enhance or modify the properties of other promoters.
Relevant Literature
Reddy and Poovaiah, Plant Mol. Biol., (1990) 14: 127-136, reports on the cloning of a cDNA for an auxin repressed mRNA from strawberry. Wilkinson, et al., Plant Mol. Biol., (1995) 27:1097-1108 report the identification of mRNAs in strawberry with enhanced expression in ripening fruit, for example RJ39.